1. Field of the Invention
The present invention relates to a nitride semiconductor device, and more particularly, to a nitride semiconductor device improved in emission efficiency due to an active layer having an optimal structure of quantum barrier and quantum well layers, notably, when operating in a high current.
2. Description of the Related Art
In general, a nitride semiconductor is broadly utilized in a green or blue light emitting diode (LED) or a laser diode (LD) which serves as a light source in a full color display, an image scanner, various signal systems and optical communication devices. This nitride semiconductor device may act as a light emitting device including an active layer emitting light of various colors such as green and yellow by virtue of recombination of electrons and holes.
Since development of the nitride LED, technological advancement has been made remarkably to broaden the scope of application of the nitride LED. Accordingly, the LED has been significantly researched as a light source for general lighting. Particularly, conventionally the nitride light emitting device has been mainly used as parts employed in a mobile product of low current and low output. However, recently, the nitride light emitting device has seen its application expanding to the high current and high output field. This has led to an urgent need for developing an LED structure with high efficiency in a high current.
FIG. 1 is a cross-sectional view illustrating a conventional nitride semiconductor device.
Referring to FIG. 1, the nitride semiconductor device 10 includes a sapphire substrate 11, and an n-type nitride semiconductor layer 12, an active layer 15 of a multiple quantum well structure, a p-type nitride semiconductor layer 17 and a transparent electrode layer 18 formed sequentially on the sapphire substrate 11.
The n-type nitride semiconductor layer 12 is partially etched to provide an area for forming an n-electrode 19a. A p-electrode 19b is formed on the transparent electrode layer 18.
Here, the active layer 15 is formed of a multiple quantum well structure having a plurality of quantum well layers 15a and quantum barrier layers 15b deposited alternately with each other.
This nitride semiconductor device has emission efficiency determined largely by recombination probability of electrons and holes in the active layer, i.e., internal quantum efficiency.
To enhance internal quantum efficiency, studies have been directed at increasing the number of effective carriers involved in light emission by improving a structure of the active layer. That is, to ensure a greater number of effective carriers in the active layer, there has been a need to reduce the number of carriers overflowing outside of the active layer.
Also, carriers are limitedly injected to a specific local area of the active layer to thereby reduce an effective active area in the total active layer. Such a decline in the effective active area directly leads to degradation in light emitting efficiency. This accordingly has called for a method for assuring recombination in the entire active layer.
A more detailed description will be given with reference to FIG. 2.
FIG. 2A illustrates a simulation result for carrier concentration of an active layer having seven pairs of quantum well layers and quantum barrier layers with a thickness of 30 Å, and 150 Å, respectively in a conventional nitride semiconductor device. FIG. 2B illustrates a simulation result for rediative recombination rate of an active layer having seven pairs of quantum well layers and quantum barrier layers with a thickness of 30 Å, and 150 Å, respectively in a conventional nitride semiconductor device.
First, according to the carrier concentration (electrons indicated by a dotted line and holes indicated by a solid line) shown in FIG. 2A, the holes are relatively less mobile than the electrons and thus with increase in the number of pairs, the holes are far less likely to survive. With a greater distance of the electrons and holes from the n-type and p-type nitride semiconductor layers, respectively, the electrons and holes are less distributed. But the holes are relatively more rapidly decreased. Thus, as shown in FIG. 2B, effective recombination probability is shown high in a quantum well layer located in an area II near the p-type nitride semiconductor layer.
The effective recombination probability of the active layer as described above may be much further decreased notably when utilized in a lighting device requiring a high current. Therefore, this has led to a need in the art for a multiple quantum well structure capable of increasing emission efficiency when the light emitting device operates in a high current.